1. Field of the Invention
The present invention relates to a method to determine the basis weight of the top ply or liner in a fibrous web, as well as to a method to regulate the basis weight.
2. Description of the Related Art
Cardboard usually consists of a top and a bottom liner between which one or more intermediate plies are located. A liner is normally produced from bleached cellulose or from other high-grade fibers and is responsible for the appearance and the printability of the cardboard. The intermediate ply is normally produced from a low grade fiber stock.
Because a show-through of the intermediate ply considerably impairs the appearance and the printability of the cardboard, the liner must have a basis weight sufficient to prevent the intermediate ply from showing through. However, a liner basis weight which is too high contains too great a component of high-grade fibers, resulting in higher associated costs.
Various methods for the determination of liner basis weights have been previously suggested. Among them is a method that provides for the basis weight to be determined by way of taking a brightness measurement, in accordance with TAPPI T-452. With this method, the relative reflection from the liner is measured at 457 nm. With this method the reflection signal is strongly dependent upon brighteners and fillers in the liner. In spite of a high TAPPI brightness the liner may display insufficient opacity and may inadequately conceal the intermediate ply.
Another method is the determination of the CIE brightness of the liner. This method takes measurements over the entire visible spectrum. It is however, also dependent upon brighteners and fillers in the liner.
What is needed in the art is a more dependable, less expensive determination of the basis weight, whereby the basis weight is determined almost independently from the influence of brighteners or fillers.